When you format your document, the effects are encoded in-memory where you have full freedom to apply any as you like.
Saving the document means translation into an external format. The external format is based on conventional primitives and not all combinations of formatting effects can be exactly converted. Writer is based on ODF standard and created .odt files. Word abides by an internal undisclosed Microsoft standard to give .doc(x) files. This latter standard has been partially retro-engineered, but this does not explain all.
Both standards are not identical. They have a non-void intersection (which accounts for the interchange) and each one has its own idiosyncrasies due to their diverging primitives (which accounts for the kind of phenomenon you experience). Among the divergences, there is the styling system which is much more developed in Writer than in Word. Consequently, you can’t translate the subtleties resulting from character styles or list styles. Of course, best efforts are attempted to approximate the effects (like using direct formatting instead of styles) but some features can’t.
The golden rule, valid for any application is “always work in the native format”. When time comes to deliver the final work to someone who can’t read .odt files, save a copy as .doc(x) and forward this copy and some reference file like .pdf so that differences can be spotted. Note that .doc, though older, is lore reliable than .docx. Also, Word claims it can read ODF files but M$ voluntarily added non-conforming features to ODF and implemented some differently.
That said, your described structure is relatively simple and should be translated without problem. Can you attach an anonymised version of your file to your question for examination?